Digital Audio Broadcasting (DAB) is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM In-Band On-Channel (IBOC) DAB can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog modulated AM or FM signal, or in an all-digital format where the analog signal has been eliminated. IBOC DAB requires no new spectral allocations because each DAB signal is transmitted within the spectral mask of an existing AM or FM channel allocation. IBOC DAB promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
One hybrid FM IBOC DAB system places orthogonal frequency division multiplexed (OFDM) sub-carriers in the region from about 129 kHz to 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier. Some FM IBOC DAB options permit subcarriers starting as close as 100 kHz away from the center frequency. The bandwidth of the existing analog FM signal is significantly smaller than the bandwidth occupied by the OFDM subcarriers. In an all-digital FM IBOC DAB system, the analog modulated host FM carrier would be eliminated. Additional digitally modulated sub-carriers may be placed, then, in the band from which the analog FM carrier is removed. Examples of hybrid and all-digital FM IBOC DAB signals are shown in U.S. Pat. No. 6,430,227.
One hybrid AM IBOC DAB system places orthogonal frequency division multiplexed (OFDM) sub-carriers in the region from about 5 kHz to 15 kHz away from the AM center frequency, both above and below the spectrum occupied by an analog modulated host AM carrier, and another set of sub-carriers in the region occupied by the analog modulated AM carrier. The bandwidth of the existing analog AM signal is significantly smaller than the bandwidth occupied by the OFDM subcarriers. In an all-digital AM IBOC DAB system, the analog modulated host AM carrier would be eliminated. Examples of hybrid and all-digital AM IBOC DAB signals are shown in U.S. Pat. No. 6,243,424.
The previously broadcast analog modulated AM or FM signal is referred to herein as a legacy signal. Receivers must provide for the reception of the larger bandwidth IBOC DAB signal, while still maintaining the existing analog performance.
Combining legacy and advanced radio receivers technologies, poses several technical challenges. Potential areas of conflict involve aspects such as combined signal filtering and intermediate frequency signal gain control.
The introduction of IBOC DAB technology changes the legacy receiver signal processing chain. The legacy analog signal occupies a relatively narrow band, while the IBOC DAB related signal is present over much broader band. An IBOC DAB enabled receiver, which is based on existing components and includes IBOC DAB handling capability, has to deploy a relatively wide band filter, which at times may affect the reception and processing capability of the legacy signal. The combined receiver may encounter this problem when either the desired station broadcasts legacy analog signal only, or when the legacy analog signal is included in a hybrid IBOC DAB signal and is selected by the receiver.
This invention seeks to provide a method and apparatus for receiving an IBOC DAB signal, without adversely impacting the analog performance.